My subwoofer recently started to have a constant "hum" coming from it. I've had the sub for about a month and I haven't noticed it until recently. The best I can make of it is it is receiving a constant signal telling it to emit a sound, as the green power light in the back stays on even after I turn the receiver off (it does sometimes eventually turn itself to sleep mode (the light turns to red), but not for a while, and sometimes not at all). The "hum" actually gets louder when I turn the receiver off. The sub is the EP350 and I have it connected to a JVC receiver using a subwoofer cable I bought at Radio Shack.

Any thoughts on what might be causing this? Thanks in advance for any help you can provide.